During the Millenium, the earth will exist in a terrestrial state. After Satan is finally cast out, the earth is ready to become celestialized. This is done with the purifying fire spoken of by Zenos. After the earth is burned with fire there will be a new heaven and a new earth: for the first heaven and the first earth were passed away (Rev 21:1).
“Evil Fruit Shall Again Come into My Vineyard”
After the Millenium, Satan is loosed for a season. He gathers his forces and has one last, great, and final conflict with the Lord:
'And when the thousand years are expired, Satan shall be loosed out of his prison,
And shall go out to deceive the nations which are in the four quarters of the earth, Gog and Magog, to gather them together to battle: The number of whom is as the sand of the sea.
And they went up on the breadth of the earth, and compassed the camp of the saints about, and the beloved city: and fire came down from God out of heaven, and devoured them.
And the devil that deceived them was cast into the lake of fire and brimstone, where the beast and the false prophet are, and shall be tormented day and night for ever and ever.' (Rev 20:7-10)